This invention relates generally to gas turbine engines and more particularly to threadless air duct connections for such engines.
A turbofan gas turbine engine used for powering an aircraft in flight typically includes, in serial flow communication, a fan, a low pressure compressor or booster, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. The combustor generates combustion gases that are channeled in succession to the high pressure turbine where they are expanded to drive the high pressure turbine, and then to the low pressure turbine where they are further expanded to drive the low pressure turbine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and the booster via a second rotor shaft.
The first rotor shaft is typically made up of a number of sections including a compressor rear shaft that is coupled to the high pressure turbine rotor and a compressor forward shaft. The compressor rear shaft includes a rear cylindrical portion and a forward conical portion. The forward edge of the conical portion is connected to the last stage disk of the high pressure compressor. A tubular air duct extends between the compressor forward shaft and the compressor rear shaft. The air duct has openings formed therein for admitting air bled from the fan or the booster, which is then ducted downstream through a bore defined by the cylindrical portion of the compressor rear shaft to pressurize an aft sump.
In one conventional arrangement, the air duct is connected to the compressor rear shaft by a threaded connection. The air duct has external threads that are threaded and tightened into mating internal threads formed in the bore of the rear shaft. However, during engine operation, particularly in the take-off portion of a flight, the compressor rear shaft grows radially more rapidly than the air duct due to its loading and thermal environment. The thermal expansion is particularly acute at the threaded joint because of its proximity to the conical portion of the rear shaft, which expands rapidly because of the relatively steep angle of the cone. This differential growth causes the threaded joint to loosen, which can lead to motion in the joint and subsequent damage and cracking of the threads. This threaded joint configuration also concentrates vibratory and bending stresses in the air duct, which can lead to fatigue failures.
Accordingly, there is a need for a threadless air duct coupling that can withstand differential thermal expansion while maintaining support of the air duct.